SPR sensors have been widely used in a variety of disciplines such as chemical, biochemical, biological, biomedical analysis, pollution monitoring, and process control.
SPR is the result of optical excitation of a surface plasmon wave (SPW) along an interface between a conducting material and a non-conducting material. A common technique for their creation is to direct a beam of electromagnetic radiation into a glass prism with an angle of incidence above the critical angle so that it undergoes total internal reflection. The internal reflection creates an evanescent electromagnetic wave at a region outside of the prism adjacent to the surface. When a thin conductive film such as gold or silver is deposited on the surface of the prism, surface plasmons will be formed.
Various types of optical sensors relying upon SPR measurement have been reported. These sensing techniques are primarily concerned with analyzing the angle, wavelength or phase properties of the reflected beam to extract the SPR information (Sensors and Actuators B, 54, 3-15, 1999). There are two most popular sensing schemes. One is the angular interrogation scheme which involves a monochromatic light source and measuring the intensity variation of the reflected beam at a range. The other is the wavelength interrogation scheme which uses a broadband light source and obtains SPR information by observing the spectral intensity variation at a fixed illumination angle.
In fact, SPR affects not only the intensity of the reflected light beam but also its optical phase at the same time. Researchers including us have found that the phase response has a steep slope near the SPR absorption dip (Optical Communication, 150, 5-8, 1998). Based on this property, phase interrogation has been estimated to provide extremely high sensing accuracy.
The first practical SPR phase measurement system was based on heterodyne interferometry (Sensors and Actuators B, 35-36, 187-191, 1996). It used an acousto-optical modulator (AOM) to modulate the signal in high frequency at 140 MHz. In order to obtain the phase information, a local oscillator was employed to shift the AOM modulation frequency at 10 kHz so that a phase meter may be employed to measure the phase difference between the reference and the probe signals. The paper describes that the estimated sensitivity, because of SPR phase measurement, has three times improvement compared to the conventional scheme. Although this sensing scheme can extract the SPR phase information from the reflected beam, the design is rather complicated both in the optical and electronic sections. In the optical part, it requires very precise optical alignment when the two optical beams are recombined again to ensure formation of detectable interference fringes. In the electronics, there are also many high frequency mixes for processing the signal. In addition, the need of acousto-optical modulator (AOM) also inerrably increases system complexity as well as costs.
Later Guo et. al. (Applied Optics, 37, 1747-1751, 1998) demonstrated a much simplified heterodyne phase sensing system using a frequency-stabilized Zeeman laser. In their system the self-frequency shift between the s- and p-polarizations due to the Zeeman's effect is employed so that the s- and p-polarizations may interfere with each other. Thus, a beat signal, which appears at the photodetector, provides the phase quantity associated with the SPR effect. The beat frequency signal ranges between tens of kilohertz to several mega-hertz. Such a high frequency is too fast to image analysis except using expensive high-speed CCD cameras. Therefore, this technique may only find applications in single sensor instruments.
More recently, a static Mach-Zehnder interferometer has been used by Nikitin et. al. (Sensors and Actuators B, 85, 189-193, 2000), for performing two-dimension SPR phase imaging. The main drawback of this design is that the system is very sensitive to mechanical movements in the optical components. Small mechanical vibrations in the mirrors or variations of temperature will inevitably cause the optical beam to move and thus leading to phase measurement error (Review of Scientific instruments, 73, 3534-3539, 2002).
It has been reported that ellipsometric measurement (Sensors and Actuators B, 51, 331-339, 1998) can also provide SPR phase information. But the drawback is that this technique involves rather cumbersome procedures. Ellipsometry equipments, whether using white light or a laser beam, are very slow machines in which several mechanical components including the polarization analyzer, the wavelength spectrometer and the goniometers are required to change position mechanically in order to obtain information. For SPR applications, which usually require real-time signal reporting, the slow speed from commercial ellipsometers is a major disadvantage.